1. Field of the Invention
This invention pertains to the calculation of the refractive index of the atmosphere and in particular to the use of a light detection and ranging (LIDAR) device to make such calculations.
2. Description of the Related Art
Knowledge of radio refractivity within the atmosphere is necessary to accurately predict electromagnetic radio propagation. For example, in radar applications it is widely known that gradients in the radio refractivity of the atmosphere can create ducts which guide radio propagation and holes in radio coverage through which objects may travel undetected. Plots that illustrate these radio refractivity characteristics are known as refractive index profiles.
In the past refractive index profiles have been assembled through the use of balloon radiosondes and aircraft-carried refractometers. These instruments provide accurate assessment of factors often used to judge radio refractivity such as relative humidity, temperature and pressure. Both of these methods, balloon radiosondes and aircraft-carried refractometers, are costly, time consuming and may be unsuitable in hostile environments.
Knowledge of relative humidity is essential to the accurate calculation of atmospheric radio refractivity. It is known that gradients, or changes in relative humidity, will produce gradients in the radio refractivity of the atmosphere. These gradients create the ducts and holes in radio propagation mentioned above.
In the past, light detection and ranging (lidar) instruments have been used to study atmospheric conditions, including relative humidity. Compared to radiosonde launchings or aircraft flights, using a lidar can be a much less costly, more time efficient and covert means by which characteristics of the atmosphere can be examined.
A lidar is an instrument that projects a laser light beam and that receives backscattered light returned from objects in the path of the projected beam. Lidars measure the time elapsed from light transmission to reception so that the range at which the projected light beam is backscattered can be determined.
The use of lidars to examine relative humidity has bee based upon the strong relationship between atmospheric backscattering and aerosols present within the atmosphere. As relative humidity increases, condensation of water vapor on the water-soluble aerosol particles causes their sizes and consequently their backscattering cross-sections to increase.
Techniques of using lidar instruments to assess atmospheric relative humidity have, however, resulted in relative humidity predictions that deviate substantially from actual measurements. This less-than-desirable result is chiefly due to an insufficient knowledge of the precise relationship between atmospheric conditions and aerosol characteristics. This has resulted in the making of erroneous assumptions such as those pertaining to atmospheric temperature, pressure, backscatter and extinction. Further exasperating these problems are inaccuracies in lidar measurements, such as those due to lidar readings taken at too large of altitude intervals.
As a result of these shortcomings, lidar-deduced relative humidity measurements have not been considered for radio refractivity calculations.